I like some of the ideas in here. Certainly work on what expansions are available or needed would be nice as they are a direct copy from the US game, and even those are a copy themselves so not every department is used.
If helipads were to be added, it would be very expensive to retrofit hospitals with them. 24 of my hospitals have helipads whereas 25 don't (8 of those are within Greater London). I've been doing some work recently to work out which hospitals have helipads. In the following areas I've listed the percentage of hospitals that have helipads, regardless of major trauma status. SECAmb (70%), EEAS (47%), SWAS (66%), LAS (11%). London is the outlier as it's so densely populated and built up that it doesn't have a need for helipads as there are so many hospitals in close proximity and even one of the MTCs there lacks one - and that's not the only MTC in the country that doesn't have a helipad.
Some hopsitals provide specialist services without the need or capacity to be a MTC so they may be better units to fly a critical patient to rather than straight to an MTC that could be even further away.
As for the MTC idea in game, it has been planned and put forward and we were expecting it to have been released already but as you can see this is not the case. They would have the capability to have extra beds from what normal hospitals in game have. On that topic, the clinic could do with a rename to Urgent Treatment Centre but this is not a priority and may be able to be done in a hospital update in the future.
With regards to the trauma unit being a specific hospital building in game, this is unnecessary as, according to sources on the NHS website, the vast majority of hospitals have a major trauma department. Obviously this doesn't make them a major trauma centre as there are further criteria they need to meet but they can still deal with most common injuries.
Ambulance stacking at hospitals is something that was requested a long time ago. It's 'on the list' but I don't know how easy implementation is with the current transport system but really we need the better variation in hospitals first before we start stacking - and a different approach to how patients are released rather than one an hour. In my opinion, releasing a patient after 1 hour of being in the emergency department would be better rather than simply one person on the hour, every hour. Keep in mind that these patients aren't necessarily being discharged, they are being admitted onto other wards for further care.